Aug 7, 2002 (CIDRAP News) – Restaurant food that was contaminated, probably deliberately, with a pesticide made 107 people sick and puzzled epidemiologic investigators for 4 months in 1998 and 1999, according to a report in today's issue of the Journal of the American Medical Assocation.

The episode at a Thai restaurant in California was blamed on methomyl, a carbamate pesticide that had been mixed into salt, says the report by Udo Buchholz, MD, MPH, then an epidemiologist with the Centers for Disease Control and Prevention (CDC), and others. Twenty-four restaurant customers who became ill from the chemical sought medical treatment for dizziness and gastrointestinal symptoms, but none were hospitalized.

"To our knowledge, this is the first large outbreak of chemical food poisoning with probable intentional background reported in the United States," the report states.

Investigation of the outbreak began Dec 21, 1998, when the Fresno County Health Department was notified that eight people had experienced nausea, vomiting, and dizziness within 2 hours after eating at the restaurant. An initial restaurant inspection and lab tests were inconclusive. On Jan 2, 1999, 11 people who had eaten at the restaurant reported the same type of illness as seen earlier. This led to a thorough investigation by county, state, and CDC officials.

In a case-control study, investigators compared people who became ill with those who stayed healthy after eating at the restaurant between Dec 20, 1998, and Jan 2, 1999. Among 317 people who were interviewed, 132 reported illness, and 107 of these met the case definition. The investigators also analyzed the restaurant menu, food preparation practices, and numerous other factors.

Methomyl, a highly toxic pesticide, was eventually found in a sample of vomitus and in salt taken from containers in the restaurant storeroom and from a stove-top salt dispenser. Investigators estimated that the dose sufficient to cause illness in 50% of those exposed is 0.15 mg/kg.

In comparing restaurant staffing patterns with the timing of illness cases, investigators found that cases were more likely when one particular cook (on a staff of 12) was on duty (odds ratio, 10.4; 95% confidence interval, 1.2 to 157.4). They were unable to determine how contamination could have occurred unintentionally. On the basis of the epidemiologic findings, Fresno police launched an investigation that was still ongoing when the report was written, though no arrests had been made.

The authors say they were not focusing on pesticides in their lab testing early in the investigation. "It was difficult and time-consuming to find a laboratory willing and able to do more specific tests for pesticides, including methomyl," they add. "Ultimately, it was not until 4 months into the investigation that we identified the responsible toxic agent." They conclude that improvement of the public health system should include strengthening of laboratory and public health capacity to recognize and control toxin-related outbreaks.